I might have a bit too much saltwater in my brain. I'd like to discuss (can you do that in this format?) the ideas of combative mermaids on land. If you say my other questions, you say my bring up Tactics Ogre Knight of Lodis as a cool inspiration for this.

The focus here is land combat, but sea combat may come up as it will effect mermaid military culture.

Movement

Crawl

As was delved into in the linked question, mermaids aren't the most agile. Their crawling speed would be in the neighborhood of 5, 8, maybe 10mph. To compare, the average human runs (sprints?) at about 15mph. However, they need to use their arms to crawl like this, making it combat ineffective. They might be able to hold a weapon while doing this, at least. Crawling with one hand or even crawling backwards is probably possible, and you can use a weapon while doing this, but it would not be easy.

Slither

Mermaids could also move with just their tails, while their human upper-body is upright. However, this is much slower. Still, it will serve as a pretty stable platform for fighting, possibly more so than a human's stance.

Quick Step: They could, in fact, curl up their tail, and make one "quick" "step". A little like a snake coiling before a big movement, but much less so.

Sidewind

Mermaids might also be able to do something a little like this, but again much less so. It does mean their sideways motion will be faster than their backward and forward motion, when moving with just their tail.

Spring

Due to their light weight, and presumably very strong tails, mermaids will likely have a very strong jump, where they can spring at you or into the air. That is, if they can get that energy shaped into a proper jump. This could be useful in a few ways.

Roll

Like people, mermaids could make use of a log roll. They might even be able to tumble forward or backwards, with some practice... possibly. This can be useful for quick, desperate, defensive motion.

Spears

These will likely be the most popular weapon with mermaids. They give them reach, which helps to make up for their lack of mobility, and are pretty useful underwater as well (so they'd have practice with them). They can also be thrown?

Thrown

I don't see why not. The mermaids have the upper body for it. While you can't get a run up, then might even be able to snap their bodies with the throw to give it a bit of extra velocity (so it might be more than a standing throw for a human, but less than a human with a run up?). Of course a mermaid would want more than one spear if they're going to be chucking them, and carrying around a bag of spears is rather cumbersome.

Tail

The tail itself is actually a reasonable weapon, for the mermaid. If they swat you in the back of the leg with this, you will fall down. If they smack you in the head with this, you will surely feel it. They can swing it around quickly, and will be rather erratic if panicked.

I can imagine mermaids creating fight-book style techniques, where after parrying a certain blow, you come around and hit them with your tail

Ranged Weapons

Just about all ranged weapons should be fine, except ones like the crossbow that requires you to use your foot to load it. The issue is, mermaids will not have much underwater practice with ranged weapons. You can use a bow or crossbow underwater, at very short ranges. Some mermaids may have practice hunting near rivers, where they come out of the water with some ranged weapon then clock an animal who was taking a drink. Being submerged underwater will be tough on their bows, however, if they use those.

Unless a mermaid commits a lot of effort into learning ranged weapons, it will be trouble, as they will not be used to the physics of projectiles.

Physiology

Posture

Mermaids would be able to assume some postures not practical for humans. Mermaids are bottom heavy, their tail probably weighs a lot compared to the rest of their body. It will also be rather long/large. This means they have a solid foundation to lean from.

Try leaning as far forward as you can, you'll notice your balance starting to give out. Bend in half like you're bowing to the King, and you can walk around and do stuff (like run into things) with reasonable balance; though taking a swing at someone would likely not go well.

Mermaids can likely take these awkward postures easily. A mermaid might be able to lean so far, their body is close to horizontal by the time

Reach

Aside from the reach with the spear, Mermaids may have an amount of reach with their posture. For example, if they "coil" up and crouch down, it will be hard for a human swordsman to reach them as you have less reach down low. But then, when they need to, the mermaid could extend her entire body into a thrust. A small mermaid is probably about six feet long, others being longer still.

As mentioned above with posture, the mermaid might be able to get close to horizontal, which gives them a much longer reach. A human can reach something like 3 to 5 feet if they really lean into it (dangerously). A six foot mermaid would have a reach of almost 8 feet! They may also be less endangered of overreaching, while doing that. What's more, due to how very low to the ground they'll be, to humans it'll feel more like 10 feet. This will make the mermaid with a spear the equivalent of melee artillery. It is also likely to make for a very strong thrust.

One disadvantage to this reach, is it would take a moment for a mermaid to retract herself from this posture. For one thing, the amount of energy and mass involved, it's going to take a moment to fully stop(dissipate the energy) so you can start retracting. You wouldn't want to be thrusting like this constantly.

As a point of interest, since mermaids are used to fighting vertically underwater, fighting horizontally likely won't seem unusual to them.

Prone and "Standing"

If you trip a human, it takes them a moment to re-orientate themselves and get up. Aside from the difficulties of how to trip a mermaid... they could swap between prone and "standing" positions quite easily. If they need to pick up loose change they dropped, easy. If you push them over, they can spring "back to their feet" like a wobbly man.

Armour

Armouring the lower parts of the mermaid's body is tricky, and normally mermaids would wear pretty limited armour underwater. You could combine armour with floats underwater, and take off the floats on land.

They could wear chainmail skirts over their tails. The upper body can mostly be armoured as a humans can be. Of course, if the upper body is too armoured, certain postures will become difficult.

Mermaids would likely wear some armour just so crawling around is less painful, possibly with a thick textile fitted to their tails to prevent chafing. But combat armour, as much as other people avoid it due to discomfort, mermaids would be far more concerned about that. They would already feel weighed down and uncomfortable on land.

Shields

Mermaids would not normally use shields underwater, as they are a pain to swim and maneuver with in water. They may try to learn their use on land, to make up for their limited mobility. Shields aren't difficult to learn the basics of, even if mermaids aren't skilled in their use it should be helpful.

However, that is an extra thing to carry when travelling, so mermaids would likely not be keen on these by and large, except maybe bucklers.

In case of armour and shield, of course, most mermaids would travel by raft, carriage or horse, so weight concerns are less of a problem.

Important point: Since mermaids can make themselves very low to the ground, they could get complete coverage from a shield (as long as no one fires from the side, as that long tail is a target).

Conclusion

Mermaids are truly unorthodox enemies to face. Their oddities and advantages don't necessarily indicate they have equal ground with human fighters on land, but there is enough evidence to suggest they would not be completely helpless.

Mermaids are very defensive fighters, using their reach to their advantage in a bid to scare off opposition. Ranged weapons are their bane, as mermaids aren't mobile, well armoured enough, nor skilled in ranged combat enough to contend with them. Still, mermaids can go prone, can move erratically, spring and roll as necessary, making them very unorthodox targets.

Mermaids could launch surprise raids out of water, from sea or rivers, then retreat back in before reinforcements arrive.

Question?

So, what are your thoughts on mermaids? Anything in this that seems incorrect, or that you would like to add? I'm sorry that this isn't a straight question, but more of a reality check and a request for second opinions.

We should consider taking this up to a chat room (I don't think I have privileges to make one, presently) to discuss it at length (I'd love to go into different types of thrusts and manoeuvres).

4 Answers
4

As far as armor goes they could use leather armor or maybe even armor created from the blubber of sea animals. ( Animal fat, especially blubber, can add a certain level of protection especially against projectile weapons).
I agree that spears are probably the mermaids' preferred weapons. As for ranged weapons there is a crab that opens and shuts its front claws so fast that it causes a gust of water so powerful that it can kill certain sea creatures. I could see the mermaids using a similar type of weapon for underwater ranged combat and hunting.

As for spears, years ago many ancient cultures carried two types of spear. First a single spear between 5 and 6 feet long for medium range combat, then they would also carry a pack of throwing spears ( 1 to 3 feet long used exclusively for throwing at targets). The smaller size made it easier to throw and allows you to carry more then one or two of them. A throwing spear might be a useful land ranged weapon.

Also remember that the mermaids are capable of taming sea animals same way we are. If they could train their killer whales they could train them to fight the same way we train them to jump and do flips in the air. Killer whales such as these might be useful for land combat since unlike most whales they have the ability to beach themselves without significant harm.

Also while it might be impractical while in water your mermaids should consider getting sheilds when they visit the land. Local merchants could make a killing off the selling shields to mermaids visiting from the sea.

It also might be interesting to give the mermaid some type of natural defense against land humanoids. Maybe something hormonal that makes people not want to fight them. Or maybe you can go to the Odysseus route and said they have such beautiful voices that once they start to sing you can't think of anything else.

$\begingroup$I expect walrus coats and suits will be popular, just for their heat retention in cold water. Making armour out of them sounds quite possible (I assume the leather can be made suitably waterproof in saltwater). Underwater, the weight of armour is far less of an issue, so long as the drag and buoyancy of it doesn't get too severe. A pistol shrimp weapon would be very neat... I'd have to work out how they could generate that much force. Javelins may well be what mermaids go for, they also are minimally damaged by water and need the less prep time to shoot. And of course, war whales!$\endgroup$
– J. DoeOct 16 '16 at 10:37

$\begingroup$One of a mermaid's best defences, is that they normally go into land in the more peaceable areas, with human escorts, so banditry and conflict are relatively rare. They also tend to travel near rivers or along the coast, so they can retreat into water when necessary. It's possible I could give them some additional magic or ability, like venomous claws or such, but I don't see the necessity presently.$\endgroup$
– J. DoeOct 16 '16 at 10:42

"Anything in this that seems incorrect..."
This one: Physiology, Posture: I have only one major issue to consider. Center of mass of a human body will be on the upper body, usually somewhere above or around the belly, so to make valid the statement that merfolk are "bottom heavy", some extra-excessive bodymass is required down there, or very heavy training for tail use to counterbalance upper body work.

Land Training: To wield combat on land is close to same difficult as to wield combat in water for humanoids (divers). Placing combat to land against land living creatures is a serious disadvantage, which is to be bridged via training, technology, or cunning. Throwing a weapon will be extremely different underwater, than on the surface, so merfolk are supposedly own much less expertise in this, and less competent in the field of firearms. So to make merfolk live to the presented expectations, they will need to put serious efforts to train themselves to land combat. This will call for secret training grounds! Some specialization might also come up similar to Spec Ops.

Gears, armor: As Bryan McClure mentioned, most convenient would be if they use armor only when reaching surface, maybe owning skills to create makeshift armors. Merfolk are using shieldpieces most commonly with strings and straps, this would pretty well fit in their style. More resilient land combat armors typically take much time to fetch and apply on wielder if not being worn at patrol duty right at the moment.

Tinkerings: Even if merfolk are not that science crazed as humans, they are still good at applying tricks and use underwater objects, creatures for their favor. If they turn out to have good tinkerers, they might figure out some pretty useful (even monumental) gadgets. Reverse Tidal Generators to raise the water level or make Assault Waves to unbalance land units near shores, Medusa Catapults, High Pressure Tubes (like firefighter hoses), Monsoon generators, poisonous creature grenades, "and much more".

Apologies if it's not done to answer old questions, but this one's quite interesting!

Movement

What movement mermaids would be capable of depends entirely on the anatomy of their tail. Do they propel themselves through the water with vertical up-down movements of their tail, or lateral side-to-side?

Because of our historical spinal structure, most seaborne mammals use the up-down stroke, with limited movement side-to-side. For similar reasons, seaborne reptiles and fish use lateral motion with limited up-down movement. Neither of these options allow for particularly efficient movement on land, and depending on the length of their tail will make moving on land exceptionally difficult (think fish or stranded porpoise).

If the tail is long enough, they should still be able to move about like seals or slither like snakes/naga (again, depending on which way their tail articulates). They will also need to have fins/flukes sufficiently shaped such that they don't get in the way (bare on the underside, not overly wide). This will make them less efficient swimmers, but it's necessary in order to make them capable of moving on land.

Make no mistake, in order to look anything like mermaids they are going to be significantly disadvantaged compared to a human moving on land.

Close Quarters Combat

I think you're dead right that spears would be favoured as you can threaten a large area of space with a spear from a stationary position, whereas at least single combat with most other weapons would be significantly hampered by the lack of mobility. Spears are also fairly hydrodynamic so would function well underwater.

However, even with a spear, the lack of mobility will be a problem. They probably won't be able to turn as quickly as a human can dart around them, and they definitely won't be able to dart backwards and forwards to stop someone closing in on them. What this probably means is that merfolk will need to fight in close order in order to be successful. This again isn't such a bad thing as this is where the spear (in combination with a shield) really shines.

Shields are a lot less hydrodynamic than spears so will probably be a later development (probably learnt from landlubbers), but will be a vital development in merfolk fighting tactics.

The shieldwall/phalanx formation also plays into one of the few benefits to merfolk physiology on land: stability. Bipedal humans trade stability for endurance (and comparatively to merfolk; speed and agility). Due to the greater mass lower down (remember, the tail has to be long) and the greater area touchign the ground, a mer-warrior will be much harder to shift backwards, throw off balance, or knock down. As most combat between shieldwalls/phalanxes/pike formations tends to descend into a push-of-pike this will be a significant advantage.

tl;dr Merfolk shieldwalls would be very difficult to defeat in close quarters.

Ranged Combat

I'm not wholly convinced that merfolk would be any good at ranged combat at all.

Humans have specifically evolved to be very good at accurately throwing things. One of the reasons we tend to be weaker weight-for-weight than other animals is that we have a disproportionate amount of slow-twitch muscles, which allow us to very accurately adjust the trajectory of thrown objects. Even compared to Neanderthals, we are more adapted to throwing things. This, along with other adaptations to vision and co-ordination, directly translates through to our proficiency at ranged warfare.

Merfolk, however, presumably evolved (I'm assuming they weren't magicked into existence) in a very, very different environment. It's very difficult to throw things underwater due to the drag both on your arm as you throw something and on the projectile itself. Bows don't function well, slings don't function well, atlatls don't function well, crossbows don't function well. The only thing that really works is spearguns, and they require the production of spring-steel which is at the very earliest late iron-age stuff.

What this adds up to is that merfolk are very unlikely to have the specific adaptations humans have to ranged combat. If they are capable of throwing, it is likely to be similar to chimpanzees (strong musculature, but comparably terrible aim). This would, however, free up merfolk upper body musculature to use more fast-twitch muscles so they'd likely be stronger than us for a given muscle mass.

tl;dr Merfolk peltasts would be terrible, and archers likely non-existent.

Cavalry

Merfolk tails are not exactly suited to comfortably riding other animals. They're also not particularly suited to clambering and climbing. Thus, horses and other mounts are highly unlikely. Elephants with howdahs are more likely, but due to the climbing issue will be trickier than for people (although not insurmountable). Chariots are a possibility, but they don't appear to be as effective as individual riders. At least, they were largely phased out by the end of the bronze age. However, they would still be very useful to merfolk as they alleviate their manoeuvrability problems.

tl;dr Merfolk cavalry will likely not be up to much in comparison to humans, but will still be useful for merfolk military tactics.

Tactics

Tactically, merfolk are somewhat similar to bronze-age Greece. Hoplite phalanxes (albeit less manoeuvrable than early greek hoplites) with chariot-borne nobility and limited ranged/cavalry support. They will likely evolve into later greek/diodachi tactics of bigger pike blocks, heavier armour and longer spears (but similar to the diodachi states, heavily relying on the strength of their phalanxes rather than the combined-arms approach of Alexander).

While these are pretty solid battlefield tactics, they are vulnerable to disruption from rough terrain and a lack of flexibility. Given their screening troops would be of lower quality than human skirmishers and cavalry they would be vulnerable to the sort of tactics that the Thracians and Romans used successfully against greek hoplites and phalanxes. They may well pick up human mercenaries to fill these roles.

tl;dr Merfolk are like bronze-age greeks, but more

Logistics

Here's where the merfolk start to fall down. Due to their slow and energy-intensive movement, their armies are going to be significantly less manoeuvrable on the logistical scale. Marching to and from engagements, intercepting rival armies, maintaining their supply lines. All of these are going to be a lot more difficult on land than for humans. They can probably use rivers to move troops fast, but they're restricted to where rivers flow to (and humans use boats for the same purpose, so it's not really an advantage other than in the time to construct fleets).

They're also less able to use cavalry to alleviate this. They can use chariots, but it gets very expensive very quickly to be outfitting entire armies with chariots (even if you have ones that are closer to carts and can carry multiple merfolk).

Historically, armies on foot have been able to travel about 15 miles per day. Any army that has been able to significantly improve on that has met with considerable success. This is one of the reasons that horse-borne steppe nomads found such significant success against a wide variety of foes. Given that it's physically much more energetic for merfolk to travel on land (human locomotion is very efficient, seals/snakes less so), I doubt that merfolk armies would be able to match the 15 miles-per-day that human armies can.

It's reasonable to assume that a reduced ability to travel distance produces a roughly equivalent inverse effect as the increase in distance. Due to this, merfolk armies will be at a significant disadvantage in any arena where there is distance to be travelled over land.

In conclusion/tl;dr

On land, merfolk would likely fight in large phalanx/shieldwall blocks in order to negate their mobility disadvantage and maximise their stability advantage. They would have minimal ranged support, cavalry and skirmishing troops. This makes their formations very vulnerable, so they will likely hire mercenaries where possible.

On campaign, they will have limited staying power away from shorelines and rivers and as such are tactically inflexible. A competent commander should be able to run rings around them in any reasonably matched land conflict.